This invention is related to techniques for determining the presence of hydrocarbons in an underground formation. More particularly, the invention offers a method for determining the oil saturation of a hydrocarbon formation from samples such as drill cuttings.
Because of the cost and time involved in placing wells on test to determine if certain intervals contain hydrocarbons or brine, it is desirable to have a logging method which accurately tells the well operator whether any hydrocarbons are present, and the saturation of such hydrocarbons. This is particularly true when a wildcat well is being drilled in an area with relatively unknown geology. Generally, current techniques are often ineffective in evaluating the oil content of an underground formation prior to placing a well on test.
In some situations, electric logs do not effectively differentiate between hydrocarbons and water. Although logging techniques indicate areas which are likely to contain brine instead of hydrocarbons or less saline water, their lack of discrimination between hydrocarbons and less saline water requires educated guesses. Because an operator does not want to overlook the presence of hydrocarbons, a frequent mistake is to place a zone on test and produce water. Such tests cost money and time. But an even more costly mistake is the failure to test a zone because the zone is thought to contain water, or to test an interval bigger than the true oil interval, producing a large flux of water and a small flux of oil. It is unknown how many times such mistakes are made.
Fluorescence has been used as a logging technique for detecting oil in drill cuttings for decades. However, the method used to determine the fluorescence of samples at a rig site is a crude method which has not improved appreciably and is severely limited in its usefulness and applicability. At present, fluorescence is determined when an operator shines a broad spectrum ultraviolet light source on cuttings in the hope of seeing substantial fluorescence to indicate the presence of oil. See U.S. Pat. Nos. 2,311,151; 2,337,465; 2,459,512; 2,951,940 and U.S. Pat. No. Re. 22,081.
There are several inherent problems in current fluorescence logging which make it nonquantitative at best and misleading at worst. First, the excitation source is not concentrated in the spectral region where the oil is most likely to absorb radiation and re-emit that radiation as fluorescence. Second, the oil is quite likely to emit fluorescence at wavelengths predominantly, if not totally, unseen by the human eye. Third, the fluorescence observed by the operator is influenced by the presence of fluorescent minerals such as fluorite. Fourth, the presence or amount of oil on the surface of the cuttings samples may not be representative of the oil in the pore structure of the formation. The mud logger sees only the surface of the samples with this technique. Fifth, operators' description of such fluorescence phenomena is highly subjective. Such commonly used words as strong, weak, bright, dull, yellow, and gold prohibit any quantitative analysis of the data.
In an attempt to overcome the subjectivity of fluorescent examination of formation samples, U.S. Pat. No. Re. 22,081 disclosed a method wherein the fluorescence of samples of known oil concentrations are visually compared by a technician to the unknown sample under the same exciting UV radiation. However, this method still retains considerable subjectivity. Most importantly, the process fails to measure emitted "invisible" fluorescence below 400 nm, the spectral region where trace amounts of oil are most likely to fluoresce with measurable intensity.
U.S. Pat. No. 2,311,151, having the same inventor as the above U.S. Pat. No. Re. 22,081, discloses two improvements wherein the emitted radiation of the U.S. Pat. No. Re. 22,081 process is (1) measured by a photo-electric cell, and (2) adjusted with a formula to compensate for the variations in surface area of the samples measured. But the improved process of U.S. Pat. No. 2,311,151 as well as U.S. Pat. No. Re. 22,081 fails to measure fluorescence below the visible spectrum.
U.S. Pat. No. 4,696,903 discloses shining UV light on formation samples and visually noting the color of the fluorescence as well as taking video pictures of the fluorescence for later study. U.S. Pat. No. 4,248,599 discloses a process for determining the API gravity of oil by the use of a flame ionization detector. In this method, the volatile and pyrolyzable components of oil are vaporized. A measurement is made of the ratio of the amount of hydrocarbon vapor produced at temperatures within a selected high temperature range to the total amount of vapor produced. A ratio of fluorescence measured under two conditions is taken in conjunction with the use of the flame ionization detector.
U.S. Pat. No. 2,591,737 detects oil by subjecting drilling fluid to steam distillation and visually inspecting the product for crude oil fractions, optionally, under UV light. U.S. Pat. No. 3,205,353 uses IR and UV light to detect contamination of underground formation samples by drilling fluid.
Hemphill, W. R. et al., "Laboratory Analysis and Airborne Detection of Materials Stimulated to Luminesce By the Sun", Vol. 31 and 32, p. 724-6, North-Holland, Amsterdam (1984) discusses the use of an airborne electro-optical device used to detect the luminescence intensity of target materials at visual wavelengths above 450 nm. Suggested uses are to detect uranium, marine oil seeps, stressed vegetation, and pollution effluents.
The emission fluorescence of crude oil samples has been studied and recorded over various wavelengths, including ultraviolet wavelengths below 400 nm. Studies which have taken place at the Bartlesville Energy Technology Center have been basically "fingerprint" studies wherein the emission fluorescence of various types of crude oils has been recorded at different excitation wavelengths. This Department of Energy research was a spin-off from earlier efforts by the Bureau of Mines to try to identify crude oil by emission fluorescence for purposes of pollution control. Please see Chisholm, B. R., Eldering, H. G., Giering, L. P., and Hornig, A. W., Total Luminescence Contour Spectra of Six Topped Crude Oils, BETC/RI-76/16, a paper prepared for ERDA for the Bartlesville Eneregy Research Center in Bartlesville, Okla., November 1976; and Brownrigg, J. T. and Hornig, A. W., Low Temperature Total Luminescence Contour Spectra of Six Topped Crude Oils and their Vacuum Distillate and Residuum Fractions, BETC/RI-78/13, a paper prepared for DOE for the Bartlesville Energy Technology Center, Bartlesville, Okla., July 1978. Similar, non-published fingerprinting work of crude oils by total luminescence spectra has also been performed in unpublished work at Texas A. & M. University.
There is one recently developed process which employs fluorescent measurement to test for the presence of hydrocarbons within drill cuttings. But this process does not give an indication of viscosity or producibility. Further, U.S. Pat. No. 4,609,821 is applicable only to oil base mud drill cuttings. The cuttings are excited with a wide range of UV wavelengths and the emitted radiation is recorded over a wide range of wavelengths to produce an analytical chemical profile. This profile of intensity over multiple wavelengths of excitation and emission radiation is compared with previous profiles to determine the presence of hydrocarbons not associated with the oil base mud.
Molecular fluorescence is discussed in general in Skoog, Douglas, Principles of Instrumental Analysis, Sanders College Publishing, Philadelphia (3rd ed. 198), pp. 225-240. The reference indicates that the greatest fluorescence behavior occurs with compounds containing aromatic functional groups and offers a table which indicates the UV fluorescence wavelengths associated with numerous benzene derivatives in ethanol solution. Several analytical profiles of hydrocarbons are disclosed wherein fluorescence intensity is plotted over multiple excitation and emission wavelengths.